cdab
[C] [D]
aA(aq) + bB(aq)cC(aq) + dD(aq) K =[A] [B]U
UThus far, we have expressed the equilibrium constant in terms of the concentrations of
reactants and products, but that is because wehave considered only reactions in which all
reactants and products are in solution. However, that is not always the case, and the manner in which a reactant or product enters
the equilibrium constant expression depends
upon the state of the substance as follows:
Thus far, we have expressed the equilibrium constant in terms of the concentrations of
reactants and products, but that is because wehave considered only reactions in which all
reactants and products are in solution. However, that is not always the case, and the manner in which a reactant or product enters
the equilibrium constant expression depends
upon the state of the substance as follows:
(^) •
Substances in aqueous solution enter K as their molar concentrations.
(^) •
Substances in aqueous solution enter K as their molar concentrations.
(^) •
Substances that are gases enter K as
their partial pressures in atmospheres.
(^) •
Substances that are gases enter K as
their partial pressures in atmospheres.
(^) •
Substances that are liquids or solids enter K as one (1) because their concentrations do not change during the reaction.
†^
(^) •
Substances that are liquids or solids enter K as one (1) because their concentrations do not change during the reaction.
†^
† The amount of a pure solid or liq
uid changes during the reaction but
not its concentration. For example,
the concentration of pure water is
1 g/mL = 56 M (its density), which is unchanged during any reaction.
Example 9.10 Example 9.10
Write the equilibrium constant expression for each of the following reactions. Write the equilibrium constant expression for each of the following reactions. a) N
(g) + 3H 2
(g) 2
U
2NH
(g) 3
a) N
(g) + 3H 2
(g) 2
U
2NH
(g) 3
The reactants and products are all gases, so the equilibrium constant expression consists of their equilibrium pressures in atmospheres.
The pressure of each substance is raised to
The reactants and products are all gases, so the equilibrium constant expression consists of their equilibrium pressures in atmospheres. a power equal to its coefficient in the balanced equation.
The pressure of each substance is raised to
a power equal to its coefficient in the balanced equation.
3
(^2) NH 22
3
P NH
K =
PP
b) Ag
2 S(s)
U
2Ag
1+(aq) + S
2-(aq)
The solid reactant enters the expression as a
one (1) in the denominator. The (aq) after
Ag
1+ and S
2- indicates that these ions are dissolved
in water, so these products enter the
expression as their molar concentrations in the numerator. The two in front of the Ag
1+^
means that we must square the Ag
1+ ion concentration. Reactions like this are discussed
in more detail in Chapter 10.
K =
[Ag
1+]
2 [S
2-]
1
= [Ag
1+
(^2) ][S
2-]
c) HF(aq) + NH
(aq) 3
U
NH
1+ 4
(aq) + F
1-(aq)
As indicated by (aq), all reactants and products
are dissolved in water, so they enter as
their molar concentrations. This is an acid
-base reaction, the topic of Chapter 12.
1+
1-
4
3
[NH ][F ]
K =
[HF][NH ]
Chapter 9 Reaction Energetics
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